OpenPGP is the standard framework for cryptography.
wikipedia: "The Free Software Foundation has developed its own OpenPGP-compliant program called GNU Privacy Guard (abbreviated GnuPG or GPG). GnuPG is freely available together with all source code under the GNU General Public License (GPL) and is maintained separately from several Graphical User Interfaces (GUIs) that interact with the GnuPG library for encryption, decryption and signing functions (see KGPG, Seahorse, MacGPG). Several other vendors have also developed OpenPGP-compliant software."
wikipedia: "There are several iOS and Android OpenPGP-compliant applications such as iPGMail for iOS and OpenKeychain for Android, which enable key generation and encryption/decryption of email and files on Apple's iOS and Google's Android."
GNU Privacy Guard (GnuPG or GPG) is "free software, compliant with RFC 4880, which is the IETF standards track specification of OpenPGP. Modern versions of PGP are interoperable with GnuPG and other OpenPGP-compliant systems." GnuPG (https://www.gnupg.org/gph/en/manual.html) provides OpenPGP support for Unix/Linux platform. Gpg4win provides OpenPGP support for Microsoft Windows platform.
OpenKeychain provides OpenPGP support for the Android platform. Available from Google Play Store. OpenKeychain provides the following license to allow third-party commercial use of the openpgp-api library - Apache License 2.0.
The GitHub library Dave Morrissey: subsampling-scale-image-view is used to implement the PhotoPGP VIEWER tab, providing an efficient image display engine, and user interface to enable pan/zoom functions. This library includes the following license to allow third-party commercial use - Apache License 2.0.